Suspended substrate transmission lines having coupled center conductors

ABSTRACT

A structure is disclosed having parallel center conductors on a substrate suspended in an offset manner between a pair of ground plane conductors. Offsetting the substrate in the manner taught results in equalizing the phase velocities of the even and odd mode portions of electromagnetic energy coupled from one of the center conductors to the other center conductor.

United States Patent [72] Inventors Arlen Kent Johnson New Providence;

George Isaac Zysman, Morristown, both of NJ.

[21] AppLNo. 27,501 [22] Filed Apr. 13,1970

Brenner, H. E., Use A Computer To Design Suspended- Substrate IC s, Microwaves Sept. 68 p. 38- 43 Copy in Gr. 255

J 1 N 7 L 9 H l H 6 Y 1. m M u N M de @m 8 i ts as PA 1] 53 47 .ll.

Bell Telephone Laboratories, Incorporated The Microwave Journal Microwave Engrs. Tech & Buyers Guide- Feb. 69 p. 69 Copy in Gr. 255 Guide- Primary Examinerllerman Karl Saalbach Assistant Examiner-Paul L. Gensler Attorneys-R. J. Guenther and William L. Keefauver [54] SUSPENDED SUBSTRATE TRANSMISSION LINES HAVING COUPLED CENTER CONDUCTORS 2 Claims, 2 Drawing Figs.

ABSTRACT: A structure is disclosed having parallel center a 44 m loo /4y i 38 0 3/pl 33 W 3 H3 W d 5 Ld U 'mF 2 UN 5 55 .l. [.l

SUSPENDEDSUBSTRATE TRANSMISSION LINES HAVING COUPLED CENTER CONDUCTORS GOVERNMENT CONTRACT The invention herein claimed was made in the course of or under a contract with the Department of the Army.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to transmission line structures having parallel center conductors mounted on substrates suspended between pairs of ground plane conductors.

2. Description of the Prior Art Electromagnetic transmission lines having flat conductive elements separated by dielectric media are kno wn in the art.

One class of such lines has at least one relatively narrow conductor mounted on a substrate which, in turn, is suspended between and spaced away from a pair of parallel ground plane conductors. This class is often identified as suspended substrate, or strip, transmission lines.

Suspended substrate transmission line construction results in an inhomogeneous dielectric medium between the ground plane conductors. This inhomogeneous medium introduces problems which are sometimes troublesome. The solution to one of these problems is presented in an article Use of a Computer to Design Suspended Substrate ICs, by H. E. Brenner, in the Sept. 1968 issue of Microwaves, pages 38 through 45. This article teaches that variations in the characteristic impedance of such a structure as a function of small variations of substrate position are minimized by nominally positioning the substrate slightly off center between the ground plane conductors.

Another problem occurs when parallel conductors on a suspended substrate are used as coupled transmission lines to produce, for example, a directional coupler or a filter. In particular, a phase velocity difference exists between odd and even mode portions of energy transferring between the conductors. This is discussed in The Even and Odd Mode Capacitance Parameters for Coupled Lines in Suspended Substrate by J. I. Smith and Coupled Transmission Line Networks in an Inhomogeneous Dielectric Medium by A. K. Johnson and G. I. Zysman, both in the 1969 G MTT Group International Microwave Symposium Digest of Technical Papers, pages 324-337. To the best of the applicants knowledge, a solution to this problem has not been previously available.

SUMMARY OF THE INVENTION An object of the invention is to make substantially equal the phase velocities of odd and even mode energy transferring between two parallel center conductors of a suspended substrate transmission line structure.

This and other objects of the invention are achieved by suspending such a substrate in a sufficiently offcentered manner with respect to the transmission line ground plane conductors. In particular, the substrate is positioned to satisfy the expression le ke la ka where:

C =even mode capacitance with the substrate absent,

C =odd mode capacitance with the substrate absent,

C =even mode capacitance with the substrate present and C =odd mode capacitance with the substrate present. The

extent of offsetting required to satisfy this expression is so much greater than that taught in the Brenner article that the present invention and Brenners teachings cannot be simultaneously incorporated in the same structure.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 shows a partially disassembled directional coupler embodying the invention; and

FIG. 2 shows a cross-sectional view of the couple of FIG. 1.

DESCRIPTION OF THE DISCLOSED EMBODIMENT FIG. 1 is a view of a partially disassembled directional coupler embodying the invention. The coupler comprises a fourwalled metallic member 10. A pair of connectors 11 and 12 are mounted on the outer surface of one of the walls of member 10 while a second pair of connectors 13 and 14 are mounted on the outer surface of the opposite wall. Affixed to the bottom of member 10 is a flat metallic ground plane member 15 which forms one of the ground plane conductors of the coupler.

A substrate 16 is mounted in member 10 in a manner to be later described. Substrate 16 has a pair of strip conductors 17 and 18 mounted thereon and connected between connectors 11 and 13 and connectors 12 and 14, respectively.

A second metallic ground plane member 19 is mounted on the top of member 10. The major portion of member 19 has been cut away in FIG. 1 in order to show substrate 16. A pair of springlike members 20 and 21. are mounted on the underside and at the opposite edges of member 19 to hold substrate 16 in place.

In use, the directional coupler may be operated so that connector 11 is an input port, connectors 12 and 13 are output ports and connector 14 is an isolation port. The manner in which a directional coupler functions to achieve these results is so well documented in the art that lfurther discussion with respect to the operation of the coupler is considered unnecessary.

FIG. 2 shows a cross-sectional view of the coupler through the section line 2-2 of FIG. 1. The width of the center conductors has been identified as w, the spacing between them as g, thickness of the substrate as d and the spacings between substrate 16 and ground plane members 15 and 19 as g, and g respectively. Values for these dimensions are chosen so that le ke lu ko where C,,, =even mode capacitance with the substrate absent,

6,, =odd mode capacitance with the substrate absent C =even mode capacitance with the substrate present and C =odd mode capacitance with the substrate present.

With a dielectric constant of 10.3 for the substrate, the following dimensions and characteristics are typical of an 8.31 directional coupler built to embody the invention:

w=0.022 inch,

g=0.005l inch,

d=0.024 inch,

g,=0.00 l 3 inch,

g =0. inch,

even mode characteristic impedance 74.6 ohms,

odd mode characteristic impedance 33.2 ohms,

line vs. free space even mode velocity 0.424 and line vs. free space odd mode velocity 0.423.

As appreciated by those skilled'in the art, two couplers constructed in accordance with the above dimensions may be connected in cascade to provide a 3d composite coupler.

The statement made earlier with respect to the inability to practice simultaneously the present invention and the teachings of the Brenner article within the same structure is believed readily apparent from a brief consideration of the above dimensions and those presented by the Brenner article wherein g and g are approximately 0.042 and 0.054 inch, respectively.

What is claimed is:

1. A transmission line structure comprising,

a pair of parallel ground plane conductors,

a flat substrate having at least two strip conductors mounted on one side thereof with at least portions of said strip conductors parallel to one another to produce an electromagnetic energy coupling relationship therebetween when said substrate is suspended between said ground plane conductors, and

means suspending said substrate in a parallel relationship between said ground plane conductors with said substrate positioned sufficiently close to one of said ground plane conductors compared to the other ground plane conductor to equalize the even and odd mode phase velocities of electromagnetic energy transferring between said strip conductors.

2. A suspended substrate transmission line structure comprising,

a pair of parallel ground plane conductors,

a flat dielectric substrate having two parallel surfaces larger than its remaining surfaces,

a pair of conductive strips of equal width mounted on one of said larger surfaces with at least portions thereof parallel with respect to one another to produce an electromagnetic energy coupling relationship therebetween when where:

C =even mode capacitance with said substrate absent, C qJdd mode capacitance with said substrate absent, C =even mode capacitance with said substrate present,

and C =odd mode capacitance with said substrate present.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,62l, 478 Dated November 16, 1971 Arlen Kent Johnson and George Isaac Zysman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 27, "10's" in the title of the article should read ICs-; line 56, "offcentered" should read --offoentered--. Column 2, line 58, "3d" should read Signed and sealed this 13th day of June 1972.

(SEAL) Attest:

ROBERT GOT'I'SCHALK EDWARD M.FLETCHER, JR.

Commissioner of Patents Attesting Officer 

1. A transmission line structure comprising, a pair of parallel ground plane conductors, a flat substrate having at least two strip conductors mounted on one side thereof with at least portions of said strip conductors parallel to one another to produce an electromagnetic energy coupling relationship therebetween when said substrate is suspended between said ground plane conductors, and means suspending said substrate in a parallel relationship between said ground plane conductors with said substrate positioned sufficiently close to one of said ground plane conductors compared to the other ground plane conductor to equalize the even and odd mode phase velocities of electromagnetic energy transferring between said strip conductors.
 2. A suspended substrate transmission line structure comprising, a pair of parallel ground plane conductors, a flat dielectric substrate having two parallel surfaces larger than its remaining surfaces, a pair of conductive strips of equal width mounted on one of said larger surfaces with at least portions thereof parallel with respect to one another to produce an electromagnetic energy coupling relationship therebetween when said substrate is suspended between said ground plane conductors, means suspending said substrate between said ground plane conductors with said larger surfaces parallel to said ground plane conductors to produce spacings between said substrate and said ground plane conductors to substantially satisfy the expression Cle/Cke Clo/Cko where: Cle even mode capacitance with said substrate absent, Clo odd mode capacitance with said substrate absent, Cke even mode capacitance with said substrate present, and Cko odd mode capacitance with said substrate present. 